In general, chain drive systems, for example, a chain drive system that transmits rotation force of a crankshaft to a camshaft in a engine of a car, has a chain tensioner provided on its slack side to keep the chain tension constant.
As a conventional chain tensioner, such a mechanism is known that has a spring and a plunger incorporated in a housing and the spring provides the plunger with a repulsive force toward the outside of the housing. In the chain tensioner of this type, the plunger pushed by the spring provides the chain with tension by pushing the chain, while the chain tension is kept constant by balancing the pushing force given by the chain to the plunger with a hydraulic pressure in a hydraulic damper chamber formed behind the plunger.
In this chain tensioner, when the chain is held tense depending on the stopping posture during a standstill of the engine, the plunger pushed by the chain may sink considerably deep. If the engine is restarted at this moment, the chain shows a sudden relaxation and the plunger projects with a large stroke toward the outside. Then the hydraulic pump that applies hydraulic pressure to the hydraulic damper chamber only discharges a small amount of oil because it is in a state immediately after activation. Such an insufficient oil supply to the hydraulic damper chamber may cause air intrusion into the hydraulic damper chamber and result in unusual noises, degrading the damping performance.
In order to solve these problems, there have been presented several chain tensioners that limit the backward movement of the plunger, for example, in Japanese Patent Publication No. Hei. 3-10819, Japanese National Publication No. Hei. 9-512884, and U.S. Pat. No. 5,931,754.
However, the invention disclosed in the Japanese Patent Publication No. Hei. 3-10819 has following drawbacks.
{circle around (1)} This invention has a latching groove formed on the inner peripheral surface of the casing and restricts the backward movement of the absorbing piston by mating a stopper ring locked in this latching groove with the outer peripheral surface of the absorbing piston. However, since the latching groove is formed on the inner peripheral surface of the cylindrical casing, it is difficult to machine the latching groove with high accuracy and the manufacturing cost becomes high.
{circle around (2)} The whole unit size in the axial direction becomes large because a spring member is placed behind the absorbing piston, followed by a check valve, and the absorbing piston has a filled structure.
{circle around (3)} The stopper ring cannot be reached directly from the outside thereof and it is radially expanded only by the movement of the absorbing piston in the axial direction. Therefore, a complex and high precision groove machining is required for the piston and the casing so that the absorbing piston is not locked by the stopper ring when the absorbing piston is inserted in the casing during assembly.
{circle around (4)} Dedicated mechanism and tools such as a groove and an assembly ring are required to maintain the post-assembly initial set state (the state where the piston is pushed deepest in the casing: see FIG. 2 in the present patent publication). Thus the number of necessary components and process steps are increased.
{circle around (5)} It is difficult to separate the piston from the casing because the resister ring cannot be manipulated from the outside. Thus maintainability or the like becomes poor.
The invention disclosed in the Japanese National Publication No. Hei. 9-512884 is based on a technological concept similar to that disclosed in the above Japanese Patent Publication No. Hei. 3-10819, thus having similar problems. This invention aims at solving the above problem {circle around (4)} by realizing the initial set state with a single ring member. However, this improvement results in a more complex groove structure.
On the other hand, in the U.S. Pat. No. 5,931,754, the latching grooves in which a clip is locked are formed on the outer peripheral surface of the piston. This structure, however, needs a large installation space in the axial direction, because the latching grooves are located remote from the spring in the axial direction and the piston is not made hollow. In addition, this invention employs a two-arm U-shaped clip instead of the ring member as the member for limiting the backward movement of the piston. This clip cannot be loaded inside the housing before the piston is inserted in the housing during assembly and thus it must be inserted in between the inner periphery of the housing and the outer periphery of the piston after the piston has been inserted. Then the inner diameter of the housing opening must be made larger than the outer diameter of the clip. In this case, another ring member (second stop ring) must be installed in the housing opening to prevent the coming-off of the clip. As a result, the number of necessary components becomes large. Also the number of components and process steps grow because dedicated members (such as a stopper pin) and grooves become necessary to maintain the post-assembly initial set state.
It is, therefore, an object of the present invention to provide a chain tensioner which is compact, easy to operate, maintain and handle and low in manufacturing cost, solving the above problems posed in the prior art.
In both inventions of Japanese Patent Publication No. Hei. 3-10819, and Japanese National Publication No. Hei. 9-512884, the latching groove is formed on the inner peripheral surface of the casing and the backward movement of the absorbing piston is restricted by mating the stopper ring locked in this latching groove with the outer peripheral surface of the absorbing piston.
In the chain tensioner of this type, the resister ring slides back and forth on the surface of the latching groove as the plunger reciprocates while the engine is running. The surface of the latching groove must be finished with high accuracy in order to lower the sliding resistance and abrasion. Conventionally, the surface is finished by grinding in general.
However, since the latching groove is formed on the inner peripheral surface of the casing, there is no choice but to conduct this finishing by so-called plunge grinding (the grindstone is pushed on the surface in the radial direction with no movement along the axial direction). Thus it is very difficult to grind the latching groove with high precision at low cost.
In the chain tensioner of this type, the stopper ring slides back and forth on the surface of the latching groove as the plunger reciprocates while the engine is running. If the sliding resistance grows, the plunger cannot smoothly move back or forth and then the response, stability and reliability of the chain tensioner degrade, affecting the durability of the stopper ring.
It is, therefore, a further object of this invention to form the above latching groove with high precision and at low cost as well as to provide a chain tensioner that shows good performance in response, stability and durability by reducing the sliding resistance between the plunger and the stopper ring during the back-and-forth movement of the plunger.